It is widely known that when the operational or, in particular, the junction temperature of an LED exceeds a certain threshold temperature the LED is permanently damaged, and consequently unable to generate light. Therefore when designing an SSL unit, the thermal design must generally prevent the LEDs of the SSL unit from exceeding this threshold under normal operating conditions.
International patent application no. WO 02/47438 discloses an LED luminary system comprising means for estimating junction temperature by employing a thermal model for the LED light sources and the current input to the LED light sources. The chromaticity coordinates of the LED light sources corresponding to a desired white light are estimated based on the junction temperature, because the characteristics of the LED light sources vary with the temperature. The output brightness of the LED light sources varies exponentially, and the peak wavelength varies linearly with the variation in the junction temperature. When the peak wavelength of the light emitted by the LED varies, the chromaticity coordinates of the LED light sources also vary. Thereby the chromaticity coordinates of the mixed light obtained form the LED luminary is different from the target light when the junction temperature of the LED changes. Hence the LED luminary system comprises a controller utilising the junction temperature estimation for maintaining the target light.
Further article published in SID 00 Digest under the title “Light output feedback solution for RGB LED backlight applications”, which is considered the closest prior art, discloses a duty controller varying the duty factor (defined as the ratio between the ON-time pulse width and total pulse width period) of the driving current for an LED array, thereby ensuring that the output chromaticity is constant, and a sensitivity matrix defining the transfer function of the sensor output to LED duty factor drive current.
However neither of the documents cited above evaluate the importance of each of the controllable parameters, namely colour set-point, output brightness and junction temperature. That is, how is the overall quality of the output light of an SSL unit best maintained in the eyes of the receiver.